[IMP] mass_mailing: allow to send a mailing on a given set of records
[IMP] mass_mailing: allow to send a mailing on a given set of records

Currently mass mailing automatically computes the remaining recipients
to mail based on existing statistics and A/B testing. However in some
cases we would like to send the mailing on a given set of records. An
example of use is to use the mass mailing mechanism on records coming
from marketing activities. This commit adds this feature by adding an
optional parameter to send. If not given it falls back on the classic
mass mailing behavior.

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